Protection system for rotating anode x-ray tubes including means for measuring the anode rotational speed



Oct. 26, 1965 w. E. TEAGUE 3,214,589

PROTECTION SYSTEM FOR ROTATING ANODE X-RAY TUBES INCLUDING MEANS FORMEASURING THE ANODE ROTATIONAL SPEED Filed Nov. 21, 1962 FIGI INVENTOR.

ATTORN YS WALTER E. TEAGUE BY% I M J l 23' W United States Patent ice3,214,580 PROTECTION SYSTEM FOR ROTATING ANODE X-RAY TUBES INCLUDINGMEANS FOR MEAS- URING THE ANODE ROTATIONAL SPEED Walter E. Teague,Silver Spring, Md, assignor to Picker X-Ray Corporation, WaiteManufacturing Division, Inc., Cleveland, Ohio, a corporation of OhioFiled Nov. 21, 1962, Ser. No. 239,249 7 Claims. (Cl. 25093) Theinvention relates to a protection system for and method of energizing arotating anode X-ray tube, and more particularly to a system and methodthat permits application of the plate voltage to the X-ray tube onlywhen both the cathode filament is glowing and the anode is rotating at aproper speed.

X-rays are produced when a high velocity stream of electrons encountersa solid material. Of the special tubes designed to produce X-rays, theevacuated thermonic tube of the Coolidge type is widely used today. Inop eration of tubes of this type, electrons are boiled off the hotfilament of a cathode in a high vacuum and accelerated towards an anodeof a solid material, such as tungsten, by an electric field produced bya voltage, known as the plate voltage, applied between the cathode andanode. When the electrons strike the solid anode, X-rays are produced.The penetrating ability, or hardness, of the X-rays depends on thevelocity of the electrons striking the anode, and their velocity in turndepends on the magnitude of the plate voltage.

The medical profession now widely employs X-rays to, for example,determine abnormalities in a patients skeletal structure. been preferredin recent years because of their greater penetrating ability, reducingthe exposure time of the patient to X-rays and also reducing theradiation dosage received by the patient. When such hard X-rays areproduced, the high velocity stream of electrons bombarding the anodeheats the anode, and tends to melt and destroy it. To preventoverheating of the anode, rotating anode X-ray tubes have been devisedand are used today. Such X-ray tubes commonly have afrustoconically-shaped target surface on the periphery of a disc anode,and the anode is mounted within the tube envelope for rotation. 'Iorotate the anode, an electric motor is provided. The rotor of the motoris mounted within the tube envelope, and the anode is mounted on therotor. The stator of the motor surrounds the tube envelope and rotor. Toproduce X-rays with such a tube, first electric power is applied to thecathode filament, to heat it to the proper temperature, and to thestator of the electric motor to rotate the rotor and attached anode ofthe X-ray tube. When the rotor and anode have reached the desired speedof rotation, typically from 2,000 to 4,000 revolutions per minute, andwhen the filament has reached the proper temperature and is glowing,boiling electrons off its surface, plate voltage is applied to the X-raytube causing electrons from the cathode to bombard the anode, producingX-rays. Should electric power not be applied to the cathode, or shouldthe cathode not have reached the proper temperature, application of theplate voltage will tend to pull ions rather than electrons from thecathode filament, ionizing the X-ray tube and rendering it useless.Should power not be applied to the electric motor, or should the anodenot be rotating at the proper speed, bombardment of the anode withelectrons will tend to overheat and destroy the anode target surface,also rendering theX-ray tube useless.

Protection systems have been devised to permit application of the platevoltage to a rotating anode X-ray tube only when the anode is rotatingat the proper speed. One

For this purpose, harder X-rays have 3,214,589 Patented Oct. 26, 1965such system in use today, disclosed in US. Patent No. 3,043,957 to E. B.Graves, indirectly determines the anode speed by detecting theelectromotive force generated in the winding of the motor when the anodeis rotating and when the energizing circuit is broken. When thiselectromotive force is of the proper magnitude, which indirectlyindicates the proper anode speed, the protection system permitsapplication of the plate voltage to the X-ray tube regardless of thecondition of the cathode filament. However, although the electromotiveforce may be of the proper magnitude, yet the anode may not be rotatingat the proper speed, as occasionally occurs when the tube is tilted andthe frictional resistance to rotation of the rotor and anode increases.In this state, application of plate voltage by the protection systemwill tend to over heat and destroy the anode target surface, renderingthe tube useless.

The protection system and method of the present invention directlydetermines both the anode: speed and the condition of the cathodefilament, and permits applica tion of the plate voltage to the X-raytube only when the anode has attained the proper speed and the filamenthas attained the proper operating temperature.

The protection system for a rotating anode X-ray tube of the presentinvention comprises means for energizing a cathode filament to glow andthereby emit radiation comprising electrons and light energy, said lightenergy being commonly classified according to frequency as infrared,visible or ultraviolet electromagnetic radiation or any combinationthereof, means responsive to rotation of the anode for chopping the beamof radiation from the cathode filament into bursts, and means forsensing the bursts of light energy radiation and for permittingapplication of the plate voltage to the anode only when the bursts oflight energy radiation are sensed, and preferably only when thefrequency of the bursts of light energy radiation has reached apredetermined value. The rotating anode X-ray tube of the presentinvention includes means for producing a beam of light energy radiationfrom the glowing cathode filament, and means responsive to rotation ofthe anode for chopping the beam of light energy radiation into bursts.

The method for energizing a rotating anode X-ray tube of the presentinvention comprises the steps of energizing the cathode filament causingit to glow and thereby emit radiation comprising electrons and lightenergy, rotating the anode, sensing light energy radiation from thecathode filament, sensing the speed of rotation of the anode, andapplying plate voltage to the anode only when both the sensed radiationof the cathode filament indicates that it is energized and the sensedspeed of rotation of the anode indicates that it is rotating at theproper speed.

The accompanying drawings illustrate a preferred embodiment of theprotection system of the present invention.

In the drawings:

FIG. 1 is a longitudinal sectional view of the rotating anode X-ray tubeand a portion of its housing,

FIG. 2 is a schematic diagram of the control circuit, and

FIG. 3 is a longitudinal sectional view on a reduced scale of amodification of a portion of the structure shown in FIG. 1.

As shown in FIG. 1, the rotating anode X-ray tube 1 comprises anevacuated and sealed generally cylindrical glass envelope 2 in which ismounted a cathode assembly 3 and an anode assembly 4. The cathodeassembly 3 generally is shaped like a crank with two cylindrical ends 6and 7 separated by a fiat body 8. The longitudinal axis of end 6 of thecathode assembly is positioned coaxial with the axis of the X-ray tubeenvelope, and passes through one end of the tube envelope, the outerside of this end of the cathode assembly being sealed to the glassenvelope. The axis of the other end 7 of the cathode assembly isparallel to the longitudinal axis of the X-ray tube and is offsettherefrom by the flat body 8 of the cathode assembly. The outer endportion 9 of end 7 of the cathode is generally cup-shaped, and in thiscup is mounted a cathode filament 11. The cup-shaped end portion 9directs a beam of radiation outwardly from the cathode. Electrical leads12 of the filament pass through a seal in end 6 of the cathode assembly,along body 8, and through end 7 to the cathode filament. These leads maybe connected to a source of electric power to energize the cathodefilament causing it to glow and emit radiation when desired.

The anode assembly 4 includes a cylindrical rotor 13 mounted forrotation in the glass envelope with its axis coaxial with the axis ofthe X-ray tube and end 6 of the cathode assembly. A threaded axle orstud 14 of reduced diameter projects from the end of the cylindricalrotor adjacent the cathode assembly. A disc-type anode 16 has an opening17 at its center to receive stud 14 of the rotor, and a nut 18 isthreaded onto the stud and forces the anode against the shoulder at thebase of the stud, clamping the anode to the rotor and mechanicallylocking the anode assembly together. The disc anode has a frustoconicaltarget face 19, of tungsten for example, adjacent its outer edge,positioned in front of and opposite the cathode filament 11 to receiveelectrons from the cathode filament.

The X-ray tube is placed within a lead housing 21 containing atransparent insulating and cooling oil bath 22, and supported byappropriate supports (not shown) to direct X-rays through a window 23 ofthe housing. A stator winding 24 is positioned about an elongated end ofthe tube envelope 2 extending from the main body. In the elongated endof the tube envelope the rotor 13 is supported for rotation. Whenelectrical energy is applied to the stator winding from an appropriatesource of power, the field produced by the stator causes rotor 13 torevolve, spinning anode 16. Such an arrangement is well known in theart, and is an arrangement commonly employed with rotating anode X-raytubes.

An outwardly projecting anode terminal 26 is sealed to the elongated endof an envelope 2, and this terminal is electrically connected throughrotor 13 to anode 16. Connections from a source of plate voltage aremade to anode terminal 26 and through electrical connections 12 to thecathode assembly. Commonly, this is termed applying the plate voltage tothe anode, since the cathode assembly often is grounded. Upon completingthese connections, plate voltage is applied between the cathode assemblyand anode 16 causing electrons to flow from the cathode filament when itis glowing to bombard the frustoconical face 19 of the anode, producingX-rays. By virtue of the angular relationship between the cathodefilament and the frustoconical face of the anode, these X-rays areemitted from the anode in a direction generally towards window 23, andpass through this window to be used as desired. This operation of arotating anode X-ray tube is well known in the art, and is an operationcommonly employed with rotating anode X-ray tubes,

An opening or passageway 31 is provided through anode 16 radiallyinwardly of frustoconical face 19 and in the path of radiation from thecathode filament. When the anode is rotating, opening 31 will pass infront of the cathode filament and permit a beam of light energyradiation from the glowing cathode filament, that is, a cathode filamentcapable of emitting radiation comprising both electrons and light energyto pass through the anode and through the transparent tube envelope 2. Areflector 32 is mounted in the path of the light energy radiationpassing through opening 31 on stator winding 24 to reflect this beam ofradiation outwardly of the envelope and through a transparent window 33in the housing 21. When the anode is rotated by rotor 13 due toenergization of the stator winding 24, opening 31 will pass betweencathode filament 11 and reflector 32 once each rotation, permitting aburst of light energy radiation to pass from the glowing cathodefilament along an optical line of sight to reflector 32 which reflectsthis burst of radiation outwardly through the transparent window 33. Adepression 34 of suitable size is provided in anode 16 diametricallyopposite opening 31 to dynamically balance the anode for rotation.

A spaced grid of lead strips 35, mounted in a bracket 36, is attached tohousing 21 on the outer side of window 33 to lie parallel to thedirection of the bursts of radiation reflected from reflector 32 throughwindow 33. A photocell 37 is attached to the housing and is positionedbehind this grid to receive bursts of radiation reflected from reflector32 and produce a pulsating electric output signal indicative of thebursts of radiation. Other devices electrically responsive to radiationmay be used in place of the photocell. The grid of lead strips being somounted permits substantially all of the bursts of radiation reflectedfrom reflector 32 to pass to photocell 37, While preventing stray X-raysemitted by anode 16 from passing to the environment surrounding housing21, since the lead strips are skewed with respect to the stray X-raysthus shielding the environment from stray X-rays.

A schematic diagram of the circuitry including photocell 37 for sensingthe bursts of radiation is illustrated in FIGURE 2. The terminals of aplug 40 connect the circuitry to a source of electric power, such as theordinary 60 cycle, volt electric power readily available from anyordinary wall outlet. An ordinary filament transformer 41 is connectedacross the terminals of plug 40 to supply filament voltage to thefilaments of the vacuum tubes incorporated in the circuitry. Theterminals of plug 40 are connected to terminals 42 and 43 of a full waverectifier 44. Thus, a 60 cycle alternating potential of 110 volts willexist between terminals 42 and 43 of full wave rectifier 44. Across theother pair of terminals 46 and 47 of the rectifier is connected avoltage divider comprising resistor 48 and potentiometer 49. Acrossterminals 46 and 47 of the voltage divider will appear a full waverectified 110 volts 60 cycle potential. Resistor 48 is of a much highervalue than potentiometer 49, and thus most of the potential existingbetween terminals 46 and 47 will exist between the terminals of resistor48. The cathode of photocell 37 is connected to the slide contact 51 ofpotentiometer 49 and the anode is connected to terminal 46 of therectifier. By varying the position of slide contact 51, the potentialacross the photocell may be adjusted. A Winding 52 of a tuned reed relay53 and thyratron 54 are connected in series across resistor 48. Thecontrol grid of thyratron 54 is connected to the cathode of photocell37. The suppressor grid of thyratron 54 is connected to the cathode ofthe thyratron. Due to the potential applied to photocell 37 there willnormally be a slight current flow through photocell 37, termed the darkcurrent. This current flow through potentiometer 49 produces a biasingpotential between the control grid and cathode of thyratron 54, tendingto cause it to conduct. By adjusting slide contact 51 of potentiometer49, this biasing potential is set to a value which is just below thevalue required to cause thyratron 54 to conduct.

When a burst of reflected light energy radiation from the cathodefilament strikes photocell 37 there will be a large current flow throughthe photocell causing a large potential to appear between the controlgrid and cathode of thyratron 54, which will cause thyratron 54 toconduct until the radiation impinging on photocell 7 ceases, at whichtime conduction also will cease.

Anodes of rotating anode X-ray tubes usually are rotated at speeds offrom 2,000 to 4,000 revolutions per minute. Thus, from 2,000 to 4,000bursts of radiation will strike photocell 37 per minute, causing from2,000 to 4,000 pulses per minute, or from 33 to 67 pulses per second, topass through the winding 52 of tuned reed relay 53 and thyratron 54during normal operation of the X-ray tube. The contacts of the tunedreed relay 53 are set to close at the desired frequency of rotation ofthe anode. Thus, if the X-ray tube is designed for a speed of rotationof the anode of 3,500 revolutions per minute, the contacts of the tunedreed relay will be set to close at 3,500 pulses per minute. However, thesetting of the tuned reed relay could be so low that any pulse wouldclose it and plate voltage would be applied when any burst of radiationwas sensed. When the setting of the tuned reed relay is reached by theanode, the contacts of the tuned reed relay 53 close, closing thecircuitry of a timer 56. The timer is set for the desired duration ofX-ray exposure. When the tuned reed relay closes, timer 56 completes thecircuit to coil 57 of relay 58 causing contact 59 to close and applyplate voltage from a source of power 61 through transformer 62 to X-raytube ll. Power is applied to coil 57 for the desired duration of theburst of X-rays as determined by the setting of timer 56, and when thisdesired duration of exposure is reached, timer 56 ceases to apply powerto coil 57, and contact 59 opens removing plate voltage from X-ray tube1.

By this arrangement, both the cathode filament 11 must be energized andglowing to emit radiation, and anode 16 must be rotating at the properspeed before the tuned reed relay 53 will close and apply plate voltageto the X-ray tube.

While an opening through the anode is preferred, other means responsiveto the rotation of the anode for chopping the radiation from the cathodefilament into bursts may be substituted. For example, a polished facetmay be provided on the anode 16 to reflect radiation from the cathodefilament to a photocell positioned to receive this radiation.Alternatively, as shown in FIG. 3, stud 14 of rotor 13' could beextended a substantial distance past nut 18 and an opening 63 providedthrough this stud to permit radiation from the cathode filament to passthrough this opening in the stud to an appropriately positionedphotocell 37' or other means for detecting the radiation.

In this embodiment, no lead strips are necessary in front of thephotocell since few stray X-rays will be present on this side of theanode. This construction both simplifies the necessary modifications tothe X-ray tube structure and positions the photocell opposite X-raywindow 23 out of the operators way. As another alternative, rather thanproviding a depression 34 in the anode, another opening through theanode opposite the first opening may be provided. In the last twoalternatives, two bursts of light would be received by the photocell foreach rotation of the anode, rather than a single burst of light for eachrotation of the anode as in the preferred embodiment. Accordingly, thesetting of tuned reed relay 53 would have to be doubled. The full-waverectifier and its connections to a source of electric power may bereplaced by any other source of direct current potential as commonlywould be available in an X-ray generator apparatus.

While the preferred embodiment of the invention has been described it isto be understood that various modifi cations may be made in the detailsof construction with out departing from the scope of the invention asset forth in the appended claims.

I claim:

1. In a protection system for a rotating anode X-ray tube which includesa cathode filament, an anode, means for rotating the anode, means forenergizing the cathode filament to cause the filament to glow andthereby emit radiation comprising electrons and light energy and meansfor applying plate voltage to the anode; means directly rotatable withthe anode for chopping the light energy radiation from the cathodefilament into bursts of light energy radiation of a frequencyproportional to the speed of rotation of the anode, and means forsensing the bursts of light energy radiation to measure the speed ofanode rotation and means for applying the plate voltage to the anodeonly when said sensing means senses a predetermined minimum frequency ofsaid bursts of light energy radiation whereby said X-ray tube isoperated only when a predetermined minimum speed is exceeded.

2. The device of claim 1 wherein said means for chopping the lightenergy radiation into bursts of light comprises a hole in the anodealigned with said cathode filament.

3. The device of claim 1 wherein said anode is mounted on a stud andwherein said means for chopping the light energy radiation into burstsof light comprises a hole in the stud.

4. In a protection system for a rotating anode X-ray tube which includesa cathode filament, an anode, means for rotating the anode, means forenergizing the cathode filament to cause the filament to glow andthereby emit radiation comprising electrons and light energy, and meansfor applying plate voltage to the anode; means rotatable with the anodefor chopping the light energy radiation from the cathode filament intobursts; means for sensing the burst of light energy radiation and forapplying a plate voltage to the anode, said last means comprising meansresponsive to the bursts of light energy radiation from the cathodefilament for producing a pulsating electrical output signal; and meansresponsive to the frequency of the pulsating output signal for applyingthe plate voltage to the anode only when the frequency of the outputsignal is at least equal to a predetermined minimum value, wherebyapplication of plate voltage to the anode is permitted only when boththe cathode filament is glowing and the anode is rotating at least at apredetermined minimum speed.

5. A protection system for a rotating anode X-ray tube which includes acathode filament, an anode, means for rotating the anode, means forenergizing the cathode filament to cause the filament to glow andthereby emit radiation comprising electrons and light energy and meansfor applying plate voltage to the anode, comprising means responsive torotation of the anode for chopping the light energy radiation from thecathode filament into bursts; means for sensing the bursts of lightenergy radiation and for applying the plate voltage to the anode,including means responsive to the bursts of light energy radiation fromthe cathode filament for producing a pulsating electrical output signal;means responsive to the frequency of the pulsating output signal forapplying the plate voltage to the anode only when the frequency of theoutput signal is at least equal to a predetermined minimum value,whereby application of plate voltage to the anode is permitted only whenboth the cathode filament is glowing and the anode is rotating at leastat a predetermined minimum speed; said means responsive to rotation ofthe anode for chopping the light energy radiation from the cathodefilament into bursts comprising a passageway through the anodepositioned, as the anode rotates, to be brought into the optical line ofsight between said cathode filament and said means for producing anoutput signal during a revolution of the anode, thereby permitting lightenergy radiation to pass from the cathode filament through the openingand to the means for producing an output signal during a revolution ofthe anode.

6. A protection system for a rotating anode X-ray tube which includes acathode filament, an anode, means for rotating the anode, means forenergizing the cathode filament to cause the filament to glow andthereby emit radiation comprising electrons and light energy and meansfor applying plate voltage to the anode, comprising a passageway throughthe anode positioned to be brought in front of the cathode filamentduring rotation of the anode for chopping light energy radiation fromthe filament into bursts, means for directing the bursts of light energyradiation laterally with respect to the longitudinal axis of the X-raytube, a photocell, means mounting the photocell in the path of thedirected bursts of light energy radiation, whereby the photocellreceives the bursts of light energy radiation and produces a pulsatingelectrical output signal therefrom, and frequency sensitive circuitmeans connected to the photocell for applying the plate voltage to theanode only when the frequency of the pulsating electrical output signalis at least equal to a predetermined minimum value, whereby applicationof plate voltage to the anode is permitted only when both the cathodefilament is glowing and the anode is rotating at least a minimum speed.

7. A protection system for a rotating anode X-ray tube which includes acathode filament, an anode, means for rotating the anode, means forenergizing the cathode filament to cause the filament to glow andthereby emit radiation comprising electrons and light energy and meansfor applying plate voltage to the anode; comprising means responsive torotation of the anode for chopping light energy radiation from thecathode filament into bursts; means for sensing the bursts of lightenergy radiation and for applying plate voltage to the anode, saidlastmentioned means comprising means responsive to the bursts of lightenergy radiation from the cathode filament for producing a pulsatingelectrical output signal; means responsive to the frequency of thepulsating output signal for applying the plate voltage to the anode onlywhen the frequency of the output signal is at least equal to apredetermined minimum value, whereby application of plate voltage to theanode is permitted only when both the cathode filament is glowing andthe anode is rotating at least at a predetermined minimum speed; and astud extending outwardly from said anode; said means responsive to therotation of the anode for chopping the light energy radiation from thecathode filament into bursts comprising a passageway through said studpositioned, as the anode rotates, to be brought into the line of sightbetween said cathode filament and said means for producing an outputsignal during a revolution of the anode, thereby permitting light energyradiation to pass from the cathode filament through the opening and tothe means to producing an output signal during a revolution of theanode.

References Cited by the Examiner UNITED STATES PATENTS 2,594,564 4/52Kehrli 313- X 3,043,957 7/62 Graves 25093 3,062,960 11/62 Laser 2501033,149,257 9/64 Wintermute 25099 FOREIGN PATENTS 684,655 12/ 39 Germany.

RALPH G. NILSON, Primary Examiner.

1. IN A PROTECTION SYSTEM FOR A ROTATING ANODE X-RAY TUBE WHICH INCLUDESA CATHODE FILAMENT, AN ANODE, MEANS FOR ROTATING THE ANODE, MEANS FORENERGIZING THE CATHODE FILAMENT TO CAUSE THE FILAMENT TO GLOW ANDTHEREBY EMIT RADIATION COMPRISING ELECTRONS AND LIGHT ENERGY AND MEANSFOR APPLYING PLATE VOLTAGE TO THE ANODE; MEANS DIRECTLY ROTATABLE WITHTHE ANODE FOR CHOPPING THE LIGHT ENERGY RADIATION FROM THE CATHODEFILAMENT INTO BURSTS OF LIGHT ENERGY RADIATION OF A FREQUENCYPROPORTIONAL TO THE SPEED OF ROTATION OF THE ANODE, AND MEANS FORSENSING THE BURSTS OF LIGHT ENERGY RADIATION TO MEASURE THE SPEED OFANODE